In installations of this kind it is necessary to provide at least one air flow control apparatus, or device, for controlling the flow rate of a stream of air passing through an aperture. The stream of air may, as the case may be, be either cold air, heated air, or conditioned air. This air passage aperture may be defined, in particular, by a duct, a distribution port, or other element of the system. It is usual to provide air flow control devices in the form of pivoting flap valves, which are typically of the curtain or butterfly type. Such flap valves can be actuated, in rotation, either manually or by means of a motorized drive. Such installations having flap valves have the major disadvantage that a relatively large amount of space has to be provided for displacement of the valve member (or flap), and this contributes to the space requirement of the heating and/or air conditioning installation within the vehicle. In addition, these types of pivoting valve have considerable resistance to the air flow, so that quite powerful motorized drives are needed for their operation.
It is also known, in particular from French patent specification No. FR 2 580 791A, to employ a control apparatus or device of the type comprising a film (also referred to as a blind or curtain), which is wound on two rollers spaced apart from each other. The film is arranged so that the portion of the film exposed between the two rollers is disposed across the air passage aperture through which the flow of air is to be controlled. The film is formed with suitable apertures, so that it has open regions through which air can pass, and solid or closed regions through which air cannot pass, so that displacement of the film by operation of the rollers, change the relationship between the open and closed regions of the film and the associated air passage aperture gives selective control of the flow rate of the air through that aperture. The film can thus be displaced in either one direction or the other, by winding it further on to one of the rollers and unwinding it at the same time from the other, thereby offering various options in the regulation of the air flow.
A device of the above kind may be referred to as a "film obturator". It has the drawback that its structure is somewhat complex. Moreover, the casing of the heating or air conditioning installation has to be specially adapted to enable the film obturator to be fitted within the casing. This in turn means that the casing of the installation then has a complex structure itself. In addition, it should be noted that known types of film obturator have to be specially designed to suit each particular type of casing of the installation. And quite apart from the above, fitting of the known film obturators involves lengthy and expensive assembly operations.
In addition, known apparatuses or devices of the "film obturator" type necessitate the provision of means for synchronizing the rotational motion of the two rollers, so as to ensure that the film unwinding from one of the rollers and winding on to the other, or vice versa, is displaced smoothly and evenly. Up to the present time, such synchronizing means comprise essentially a metallic wire which is wound around the two shafts of the respective rollers, together with means for tensioning the metallic wire. Such synchronizing means are complex and require somewhat delicate adjustment.